Motion aftereffects are generally considered evidence for cell populations tuned to specific directions of motion. Despite some early reports, there is scant recent physiological or psychophysical evidence for neurons in visual cortex selective for the direction of motion through depth (i.e., tuned to 3D motion). By comparing adaptation under dichoptic and monocular conditions, we found large 3D motion aftereffects that could not be explained by a simple combination of monocular aftereffects.

Subjects viewed random dot stereograms containing corresponding dots moving in opposite horizontal directions in the two eyes, thus producing 3D motion percepts. Following adaptation to this 3D motion (30 sec, towards or away), subjects performed a series of direction discrimination trials composed of a 3 sec top-up adaptation and 0.5 sec test stimulus. On each trial, the test stimulus contained a variable proportion of signal dots moving through depth, and the remainder of the dots followed random walks through depth (i.e. motion coherence was varied, akin to many 2D motion studies). On each trial, subjects reported the perceived global motion direction (towards or away).

We observed a very strong 3D motion aftereffect. Prolonged viewing of unidirectional 3D motion biased subsequent percepts of noisy 3D motion displays in the direction opposite adaptation. The contribution of 2D monocular adaptation was dissociated by measuring monocular aftereffects after identical 3D adaptation. Surprisingly, these aftereffects were much weaker (∼4×) than the corresponding binocular aftereffects. This suggests that the effects of adapting to 3D motion cannot simply be accounted for by the summation of monocular 2D aftereffects; mechanisms tuned to 3D motion must be involved.

These results provide clear evidence for the existence of cell populations tuned to 3D direction, and suggest that 3D motion aftereffects (e.g. Sakano et al., 2005) can be used to further characterize these mechanisms.